Hybrid fiber coaxial (HFC) cable television systems have been in widespread use for many years and extensive networks have been developed. A typical HFC network generally contains a headend which provide communications between user in the HFC network and the IP/PSTN networks. The headend usually contains a cable modem termination system (CMTS) containing several receivers, each receiver handling communications between hundreds of end user network elements. The headend is generally connected to several nodes and each node is connected to many network elements, such as data over cable system (DOCSIS) terminal network elements (e.g. media terminal adapters (MTA) or cable modems), e.g., a single node may be connected to hundreds of modems. In many instances several nodes may serve a particular area of a town or city.
A typical HFC network uses optical fiber for communications between the headend and the nodes, and uses coaxial cable for communications between the nodes and the end users. Downstream optical communications over the optical fiber are typically converted at the nodes to RF communications for transmission over the coaxial cable. Conversely, upstream (or return path) RF communications from the users are provided over the coaxial cables and are typically converted at the nodes to optical communications for transmission over the optical fiber. The return path optical link (the optical components in the HFC network, e.g. the transmission lasers, optical receivers, and optical fibers) contribute to the performance of the HFC network. More particularly, the optical components contribute to the quality of signals received by the CMTS from the users, and may cause distortion of the signals or otherwise degrade their quality.
Driving the RF input power too high at the optical transmitter in the return path on a node, via either mismanagement of the active channels or ingress noise, often creates excess distortion and degrades the quality of the received signals at the CMTS. This overdriving condition is typically known as laser clipping, which may be corrected by properly managing the RF input power to the transmitter and allowing for adequate headroom to withstand ingress events.
However, the cause for the overdriving condition and laser clipping is often difficult to detect when it occurs due to the burstiness of the signals typically present on the return path. Currently, diagnosing the cause for laser clipping requires a technician or engineer to be at multiple locations within the HFC plant simultaneously with specialized test equipment, such as a vector signal analyzer and signal generators. This manual diagnostic process is labor intensive, time consuming and costly.